1. Field of the Invention
The present invention relates to a method for processing a hearing aid input signal. The invention further relates to a hearing aid and to a signal processor for a hearing aid.
2. The Prior Art
In WO 99/34642 a hearing aid having a signal processor with multiple processing channels is disclosed, in which dynamic automatic gain control is effected by detection of the input sound level and/or the output sound level and adapting the output sound level in response to the detected sound level by controlling the gain in each processing channel towards an actually desired value of the output sound level. The gain control is effected at increases and decreases, respectively, of the input sound level by adjusting the gain towards the desired value with an attack time and a release time, respectively, which in response to the detected sound level are adjusted to a relatively short duration providing fast gain adjustment at high input and/or output sound levels and to a relatively long duration, providing slow gain adjustment, at low input and/or output sound levels.
In a practical implementation of this prior art hearing aid, the dynamic gain control is effected partly on the basis of the instantaneous sound input received by the hearing aid, partly on the basis of a statistical analysis of the sound level within a time window extending 20 to 30 seconds back in time. The actual gain adjustment is calculated by a complex algorithm to determine the actual gain control in each channel and the rate of control.
This dynamic gain control has appeared to offer significant advantages compared to earlier AGC methods for hearing aid gain control. At low sound levels, at which the transfer function provides a compressor characteristic and the reproduced sound is sensitive to pumping or vibrating sound effects at varying gain, the sound will be controlled with long attack and release times, whereas at high sound levels, at which the reproduced sound approaches the clipping or pain threshold, the sound will be controlled with short attack and release times.
This prior art hearing aid has moreover been implemented with an effective noise suppression based on detection of the contents of speech and noise in each processing channel. In the absence of noise, the noise suppression or noise squelching is not effective, whereas at the occurrence of heavy noise in a frequency band the gain adjustment otherwise resulting from the dynamic gain control is modified towards a reduced gain. Thereby, the advantage is obtained that use of the hearing aid in a noisy environment in a relatively long time is made possible without causing unacceptable discomfort to the user.
In general, the use of temporary noise suppression or noise squelching in hearing aids or similar devices has been disclosed in several prior art publications.
U.S. Pat. No. 4,630,302 discloses a method and apparatus for aiding hearing with an automatic gain control unit having a first section for increasing the amplitude of input signal segments below a threshold level and a second section for reducing the amplitude of input signal segments above the threshold level. A noise suppressor unit having a long attack time and a short release time is responsive to the output from the second section of the automatic gain control unit and has a threshold level of operation below the threshold level of the automatic gain control unit to pass speech signals and squelch background noise signals between speech signal segments.
In U.S. Pat. No. 4,852,175 a hearing aid signal processing system is disclosed, in which noise squelching in each of a plurality of frequency bands is effected by estimation of the absolute quantity of noise by monitoring the amplitude distribution of sound events in each band and comparing the absolute quantity of noise in a current frequency band, in which gain is to be adjusted, with the absolute quantity of noise in a next high frequency band, whereby the gain in the current frequency band is reduced, if the noise quantity in this band exceeds the noise quantity in the next high band by more than a predetermined threshold value.
In U.S. Pat. No. 5,768,473 an adaptive speech filter is disclosed, in which frequency components of an information signal from an input signal also containing noise is effected by calculation of the total power in each frequency component, estimating the power of the information signal included therein and calculating a modified gain for each frequency band as a function of the total power, the information signal power estimate and a previous estimate of a noise power, the input frequency component being multiplied by said modified gain to produce an estimate of the power of the frequency component of the information signal and a new noise power estimate being estimated from the previous noise power estimate and the difference between the total power in the frequency component and the estimate of the power of the frequency component of the information signal, regardless of whether there is a pause in the information signal.
In the noise squelching implemented in the prior art hearing aid of WO 99/34642 the statistical noise estimation in each frequency band will result in a relatively slow gain reduction, which in case of input signals containing speech and noise components having comparable sound levels has been observed to reduce the perception and the intelligibility of speech in certain situations, e.g. when the hearing aid is used during car driving.
On this background, it is the object of the invention to provide a signal processing method and a signal processor for a hearing aid, in which the content of speech in an input signal also containing noise is intensified to improve the perception of speech.
In a first aspect, the invention provides a method for intensification of speech signals components in a hearing aid input signal including background noise, comprising the steps of classifying said input signal into at least three frequency bands comprising at least one high frequency band, a low frequency band and at least one frequency band intermediate said high frequency band and said low frequency band, estimating the level of background noise in said low frequency band and at least one intermediate frequency band and adjusting the gain in said low and said one intermediate frequency band in response to the estimated level of background noise to provide squelching of said background noise, characterized by comprising the steps of estimating the content of speech signal components in said at least one high frequency band and modifying the gain adjustment caused by said background noise estimation in at least said one intermediate frequency band to reduce said squelching of background noise and thereby intensify the content of speech signals components occurring in said one intermediate frequency band.
The invention is based on the recognition of the fact that the observed reduction in speech intelligibility referred to above is caused by the effect of upward spread or masking of noise, by which noise typically occurring in the low frequency band of the signal processing system is spread upwards to the adjacent higher frequency band, which will normally contain frequency components of significant importance for speech perception. In result, noise squelching will be effected not only in the low frequency band having the major content of noise, but also in the next higher intermediate frequency band.
The modification of the gain adjustment in this frequency band on the basis of speech components in at least the highest frequency band, in accordance with the invention, yields an intensification or an enhancement of the speech content in intermediate frequency band, which has been observed to provide significant improvement of speech perception.
Whereas the estimation of noise and speech signal components can be effected by a variety of methods known per se, such as disclosed in WO 99/34642, e.g. FFT analysis or peak detection, it is preferred for the method according to the invention that, following said classification, said input signal is subjected to digital signal processing in each of said frequency bands and said estimations of the quantity of background noise and the content of speech signal components are effected by percentile estimation.
In a second aspect, the invention provides a signal processor for a hearing aid, comprising means for receiving an input signal containing speech signal components in the presence of background noise, means for classification of said input signal into at least three frequency bands comprising at least one high frequency band (C), a low frequency band (A) and at least one frequency band (B) intermediate said high frequency band and said low frequency band, variable gain adjustment means for controlling the gain in each of said frequency bands and means for estimating the level of background noise in said low frequency band and at least one intermediate frequency band and adjusting the gain in said low and said one intermediate frequency bands in response to the estimated level of background noise to provide squelching of said background noise, characterized by comprising means for estimating the content of speech signal components in said at least one high frequency band and for modifying the gain adjustment caused by said background noise estimation in at least said one intermediate frequency band to reduce said squelching of background noise and thereby intensify the content of speech signal components occurring in said one intermediate frequency band.
In a third aspect, the invention provides a hearing aid, comprising means for receiving an input signal, means for classification of said input signal according to frequency into at least three frequency bands, designated a high frequency band (C), a low frequency band (A) and an intermediate frequency band (B) intermediate said high frequency band and said low frequency band, gain adjustment means for controlling the gain in said intermediate frequency band, means for estimating the level of background noise in said low frequency band and said intermediate frequency band, means for setting the gain in said intermediate frequency band to a trial value in response to the estimated level of background noise and adapted to suppress background noise, means for estimating the content of speech signal components in said high frequency band and means for modifying the trial value of gain in said intermediate frequency band in order to enhance the content of speech signal components present in said intermediate frequency band.
Preferred embodiments appear from the dependent claims.